In a large saucepan, cook onion and sweet pepper in hot oil over medium heat for 4 to 5 minutes or until crisp-tender. Add meatballs, marinara sauce, and crushed red pepper. Bring to boiling; reduce heat. Simmer, covered, about 15 minutes or until meatballs are heated through.
To serve, place provolone cheese on bottom halves of rolls. Spoon meatball mixture onto each roll. If desired, broil sandwiches 4 to 5 inches from heat for 1 to 2 minutes or until cheese melts. Makes 6 sandwiches.